Attention is a resource
For the past few years, a Microsoft Canada Consumer Insights Report has made the media rounds claiming that the average human attention span has declined from 12 seconds in 2000 to about 8 seconds in 2013, compared to 9 seconds for goldfish.
There are many implications to this work that are lost in the second-hand reporting, and I would encourage anyone to read the core study. For our purposes, though, I think it brings to the forefront that attention is a resource, and a highly limited one at that.